Potential applications of carbon nanotubes

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The potential applications of carbon nanotubes are varied. Carbon nanotubes, a type of fullerene, have potential in fields such as nanotechnology, electronics, optics, materials science, and architecture. Over the years new discoveries have led to new applications, often taking advantage of their unique electrical properties, extraordinary strength, and efficiency in heat conduction.

Structural

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• clothes: waterproof tear-resistant textiles
• combat jackets: MIT is working on combat jackets that use carbon nanotubes as ultrastrong fibers and to monitor the condition of the wearer.^[1] Cambridge University has developed the fibres and given a license to a company.^[2]
• concrete: In concrete, they increase the tensile strength, and halt crack propagation ^[3].
• polyethylene: Researchers have found that adding them to polyethylene increases the polymer's elastic modulus by 30%.
• sports equipment: Stronger and lighter tennis rackets, bike parts, golf balls, golf clubs, golf shaft and baseball bats.
• space elevator: This will be possible only if tensile strengths of more than about 70 GPa can be achieved.
• synthetic muscles: Due to their giant elongations and contractions when a current is run through them, CNTs are ideal for synthetic muscle^[4]
• high tensile strength fibers: A large number of research groups have spun fibers of single wall carbon nanotubes embedded into a polymer. For example, fibers produced with polyvinyl alcohol required 600 J/g to break^[5] In comparison, the bullet-resistant fiber Kevlar is 27–33 J/g.
• bridges: Carbon nanotubes may be able to replace steel in suspension bridges.
• ultrahigh-speed flywheels: The high strength/weight ratio enables very high speeds to be achieved.
• fire protection: covering material with a thin layer of buckypaper significantly improves its fire resistance due to the efficient reflection of heat by the dense, compact layer of carbon nanotubes or carbon fibers.^[6].

Electromagnetic

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• artificial muscles^[7]
• buckypaper - a thin sheet made from nanotubes that are 250 times stronger than steel and 10 times lighter that could be used as a heat sink for chipboards, a backlight for LCD screens or as a faraday cage to protect electrical devices/aeroplanes.
• chemical nanowires: Carbon nanotubes additionally can also be used to produce nanowires of other chemicals, such as gold or zinc oxide. These nanowires in turn can be used to cast nanotubes of other chemicals, such as gallium nitride. These can have very different properties from CNTs - for example, gallium nitride nanotubes are hydrophilic, while CNTs are hydrophobic, giving them possible uses in organic chemistry that CNTs could not be used for.
• conductive films: A 2005 paper in Science notes that drawing transparent high strength swathes of SWNT is a functional production technique.^[8] Additionally, Eikos Inc of Franklin, Massachusetts and Unidym Inc.^[9] of Silicon Valley, California are developing transparent, electrically conductive films of carbon nanotubes to replace indium tin oxide (ITO) in LCDs, touch screens, and photovoltaic devices. Nanotube films show promise for use in displays for computers, cell phones, PDAs, and ATMs.
• electric motor brushes: Conductive carbon nanotubes have been used for several years in brushes for commercial electric motors. They replace traditional carbon black, which is mostly impure spherical carbon fullerenes. The nanotubes improve electrical and thermal conductivity because they stretch through the plastic matrix of the brush. This permits the carbon filler to be reduced from 30% down to 3.6%, so that more matrix is present in the brush. Nanotube composite motor brushes are better-lubricated (from the matrix), cooler-running (both from better lubrication and superior thermal conductivity), less brittle (more matrix, and fiber reinforcement), stronger and more accurately moldable (more matrix). Since brushes are a critical failure point in electric motors, and also don't need much material, they became economical before almost any other application.
• light bulb filament: alternative to tungsten filaments in incandescent lamps.
• magnets: MWNTs coated with magnetite
• optical ignition: A layer of 29% iron enriched SWNT is placed on top of a layer of explosive material such as PETN, and can be ignited with a regular camera flash.^[10]
• solar cells: GE's carbon nanotube diode has a photovoltaic effect. Nanotubes can replace ITO in some solar cells to act as a transparent conductive film in solar cells to allow light to pass to the active layers and generate photocurrent.
• superconductor: Nanotubes have been shown to be superconducting at low temperatures.^[11]
• ultracapacitors: MIT is researching the use of nanotubes bound to the charge plates of capacitors in order to dramatically increase the surface area and therefore energy storage ability.^[12]
• displays: One use for nanotubes that has already been developed is as extremely fine electron guns, which could be used as miniature cathode ray tubes in thin high-brightness low-energy low-weight displays. This type of display would consist of a group of many tiny CRTs, each providing the electrons to hit the phosphor of one pixel, instead of having one giant CRT whose electrons are aimed using electric and magnetic fields. These displays are known as field emission displays (FEDs).
• transistor: developed at Delft, IBM, and NEC.

• electromagnetic antenna ^[13]

Electroacoustic

• loudspeaker: In November 2008, researchers at the Tsinghua-Foxconn Nanotechnology Research Centre in Beijing announced they had created loudspeakers from sheets of parallel carbon nanotubes, generating sound in a manner similar to how lightning produces thunder. Near-term commercial uses include replacing piezoelectric speakers in greeting cards.^[14]

Chemical

• air pollution filter: Future applications of nanotube membranes include filtering carbon dioxide from power plant emissions.^[15]
• biotech container: Nanotubes can be opened and filled with materials such as biological molecules, raising the possibility of applications in biotechnology.
• hydrogen storage: Research is currently being undertaken into the potential use of carbon nanotubes for hydrogen storage. They have the potential to store between 4.2 and 65% hydrogen by weight. This is an important area of research, since if they can be mass produced economically there is potential to contain the same quantity of energy as a 50L gasoline tank in 13.2L of nanotubes. See also, Hydrogen Economy.^[16]
• water filter: Recently nanotube membranes have been developed for use in filtration. This technique can purportedly reduce desalination costs by 75%. The tubes are so thin that small particles (like water molecules) can pass through them, while larger particles (such as the chloride ions in salt) are blocked.

Mechanical

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• Oscillator: fastest known oscillators (> 50 GHz).
• Nanotube membrane: Liquid flows up to five orders of magnitude faster than predicted by classical fluid dynamics.
• Slick surface: slicker than Teflon and waterproof.
• Carbon nanotube actuators
• infrared detector: The reflectivity of the buckypaper produced with "super-growth" chemical vapor deposition method is 0.03 or less. It is possible to apply it to the performance gain of pyroerectric infrared detector. ^[17][18]

• radiometric standard: As a standard of the black.
• thermal radiation: For the thermal emission in the space such as space satellites.
• stealth: Absorbance is high in wide ranges from FUV to FIR.

Electrical circuits

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A nanotube formed by joining nanotubes of two different diameters end to end can act as a diode, suggesting the possibility of constructing electronic computer circuits entirely out of nanotubes. Because of their good thermal properties, carbon nanotubes can also be used to dissipate heat from tiny computer chips. The longest electricity conducting circuit is a fraction of an inch long.^[19]

Fabrication difficulties are major hurdles for carbon nanotubes to find prominent places in circuits. The production of electrical circuits with carbon nanotubes are very different from the traditional IC fabrication process. The IC fabrication process is somewhat like sculpture - films are deposited onto a wafer and pattern-etched away. Because carbon nanotubes are fundamentally different from films, carbon nanotube circuits can so far not be mass produced.

Researchers sometimes resort to manipulating nanotubes one-by-one with the tip of an atomic force microscope in a painstaking, time-consuming process. Perhaps the best hope is that carbon nanotubes can be grown through a chemical vapor deposition process from patterned catalyst material on a wafer, which serve as growth sites and allow designers to position one end of the nanotube. During the deposition process, an electric field can be applied to direct the growth of the nanotubes, which tend to grow along the field lines from negative to positive polarity. Another way for the self assembly of the carbon nanotube transistors consist in using chemical or biological techniques to place the nanotubes from solution to determinate place on a substrate.

Even if nanotubes could be precisely positioned, there remains the problem that, to this date, engineers have been unable to control the types of nanotubes—metallic, semiconducting, single-walled, multi-walled—produced. A chemical engineering solution is needed if nanotubes are to become feasible for commercial circuits.

Interconnects

Metallic carbon nanotubes have aroused a lot of research interest in their applicability as Very-large-scale integration (VLSI) interconnects of the future because of their desirable properties of high thermal stability, high thermal conductivity and large current carrying capacity^{[20][21][22][23][24][25]}. An isolated carbon nanotube can carry current densities in excess of 1000 MA/sq-cm without any signs of damage even at an elevated temperature of 250 degrees C, thereby eliminating electromigration reliability concerns that plague Cu interconnects. Recent modeling work comparing the performance, power dissipation and thermal/reliability aspects of carbon nanotube interconnect to scaled copper interconnects have shown that carbon nanotube bundle interconnects can potentially offer advantages over copper.^[26] Additionally, the concept of hybrid interconnects-employing carbon nanotube vias in tandem with copper interconnects has been shown to offer advantages from a reliability/thermal-management perspective.

Transistors

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Semiconducting CNTs have been used to fabricate field effect transistors (CNTFETs), which show promise due to their superior electrical characteristics over silicon based MOSFETs. Since the electron mean free path in SWCNTs can exceed 1 micrometer, long channel CNTFETs exhibit near-ballistic transport characteristics, resulting in high speed devices. In fact, CNT devices are projected to be operational in the frequency range of hundreds of GHz. Recent work detailing the advantages and disadvantages of various forms of CNTFETs have also shown that the tunneling based CNTFET offers better characteristics compared to other CNTFET structures. This device has been found to be superior in terms of subthreshold slope - a very important property for low power applications.^{[27][28][29][30][31][32]}

Nanotubes are usually grown on nanoparticles of magnetic metal (Fe, Co) that facilitates production of electronic (spintronic) devices. In particular control of current through a field-effect transistor by magnetic field has been demonstrated in such a single-tube nanostructure.^[33]

Electronic design and design automation

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Although carbon nanotube devices and interconnects have been separately shown to be promising in their own respects, there have been few efforts to successfully combine them in a realistic circuit. Most CNTFET structures employ the silicon substrate as a back gate. Applying different back gate voltages might become a concern when designing large circuits out of these devices. Several top-gated structures have also been demonstrated, which can alleviate this concern. Recently, a fully integrated logic circuit built on a single nanotube has been reported. However, this circuit also employs a back-gate. Additionally, there are still several process related challenges that need to be addressed before CNT-based devices and interconnects can enter mainstream VLSI process. This makes it an exciting and open field for research. Problems like purification, separation of carbon nanotubes, control over nanotube length, chirality and desired alignment, low thermal budget as well as high contact resistance are yet to be fully resolved. Although these are serious technological challenges, innovative ideas have been proposed to build practical transistors out of nano-networks. Since lack of control on chirality produces a mix of metallic as well as semi-conducting CNTs from any fabrication process and it is difficult to control the growth direction of the CNTs, random arrays of SWCNTs (that are easily produced) have been proposed to build thin film transistors. This idea can be further exploited to build practical CNT based transistors and circuits without the need for precise growth and assembly.

References

Specific references:

General references:

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External links

• Lecture by Ray Baughman (YouTube)
• Applications of Carbon Nanotubes